The present invention relates to heterojunction type GaAs field-effect transistors of the type in which a gate region consists of a semiconductor layer heteroepitaxially grown upon one major surface of a semiconductor substrate.
There has been a strong demand for field-effect transistors capable of operating at high speeds in a high frequency range so that various electronic computers and control circuits may be operated at high speeds and so that such field-effect transistors may be used in amplifiers for super high frequency television systems and microwave communication systems. Recently, much attention has been devoted to field-effect transistors made of semiconductor compounds with high mobility which are selected from the III and V groups. Among them, most useful field-effect transistors are of the shottky barrier type in which the gate consists of a shottky barrier created upon a GaAs substrate. However, the shottky barrier field effect transistors have undesirable features in that it is difficult to form the shottky barrier, that the thermal stability of the shottky barrier is not satisfactory at high temperatures, and that it is difficult to attain by the conventional masking techniques the correct registration among gate, source and drain required to shorten the distance between the gate and source, and the distance between the gate and drain and to control the length of the gate in order to ensure satisfactory operation at high frequencies.
There has been also devised and demonstrated junction field-effect transistors of the type in which the p-n junction gate is formed within a GaAs substrate by diffusion. However the diffusion speed in the lateral direction along the intersurface between the diffusion mask and the GaAs substrate is faster than the diffusion speed in the longitudinal direction so that the length of the gate must be minimized as much as possible in order to provide field-effect transistors capable of operating at high frequencies. But so far there has not been proposed yet a satisfactory method for shortening the length of the gate.